1. Field of the Invention
The invention relates to switchgear in which a quenching arc is formed between electrodes, namely, to electromagnetic switchgear using magnetic blast which provides for the lengthening of the quenching arc when the arc column is acted upon by the force resulted from electromagnetic interaction between the arc current and a magnetic field. In particular, the invention relates to electromagnetic arc extinction apparatus used in switchgear.
The apparatus of the invention is suitable for use on any desirable alternating- or direct-current circuits and can find use in high-voltage heavy-current switchgear including circuit breakers, fuses and electroexplosion trips.
2. Description of the Prior Art
A quenching arc occurring in switchgear can be extinguished by recovering the arc voltage up to that existing across the contacts of the circuit being switched.
It is known that the breaking arc voltage is given by U=El.sub.1 +.SIGMA.U.sub.N, where E is the intensity of the electric field in the arc column, l.sub.a is the length of the quenching arc, and .SIGMA.U.sub.N is the sum of the voltage drops at the electrodes. There are therefore three methods by which the arc voltage can be increased: increasing the number of voltage drops at the electrodes; increasing the intensity of the electric field in the arc column; and lengthening the quenching arc.
During the switching of alternating-current circuits, the arc can be extinguished (the electric strength of the interelectrode gap can be restored) at current zero in an alternating-current circuit. This feature can be used effectively in the case of vacuum-type circuit breakers which are being developed on a wide basis at the present time.
Increasing the number of voltage drops at the electrodes is basically applicable to low-voltage switchgear. In this case, the quenching arc is split into a number of series-connected smaller arcs each of which has its own voltage drop at the anode and cathode. Since the sum of these voltage drops does not usually exceed several tens of volt this method is used as an auxiliary one in the case of high-voltage switchgear.
At present, the method of increasing the intensity of the electric field in the arc column is basically suitable for high-voltage applications where U exceeds 10 to 20 kV. The method can find use in a.c. oil, small oil volume, air-blast and SF.sub.6 -filled circuit breakers. For these voltages, a.c. circuit breakers rated for higher parameters act as d.c. circuit breakers.
In these apparatus, the intensity of the electric field in the arc column is increased in a manner that the arc column is subjected to longitudinal, lateral or radial/longitudinal blast using the working gas (compressed air or sulphur hexafluoride gas) or the products obtained during the decomposition of the working liquid (oil). In this case, the intensity E is equal to hundreds or thousands of V/cm, the spacing between the electrodes amounts to several tens of centimeter, the arc length l.sub.1 reaches one to two meters, and a maximum turn-off voltage for one pair of electrodes reaches a value of 100 to 300 kV. However, the disadvantage of the apparatus is that they have a low switching time, .tau., which is usually equal to 0.2 to 0.06 s. In the latest embodiments, attempts are made to attain 0.02 s switching time by using sulphur hexafluoride gas.
Another disadvantages of the described apparatus are concerned with large dimensions and weight and with sophisticated design and laborious maintenance. Indeed, oil is fire-hazardous, air must be compressed, and SF.sub.6 gas requires that the construction be hermetically sealed.
The lengthening of the quenching arc basically applies to electromagnetic switchgear in which case an arc is caused to move under the action of the force F resulted from electromagnetic interaction between the arc current and a magnetic field, that movement being performed over diverging (horn-shaped) electrodes and accompanied by a lengthening of the arc. The magnetic field is formed by external sources such as arc-quenching coils or the magnetic field of the arc current itself is employed.
Electromagnetic switchgear offers simple design features and good reliability, allows for multiple switching of circuits, does not require special working medium and provides for a higher operational speed that is increased with an increase of the current being interrupted, which makes this switchgear current-limiting during the interruption of short-circuit currents. Finally, with electromagnetic switchgear a.c. and d.c. uses are possible.
The switching time .tau. of electromagnetic switchgear is determined by the distance a covered by the arc column when it is moved in a direction of the driving force F, and is also determined by the velocity V.sub.a which is dependent upon the force F and upon the conditions under which the arc column is moved. Therefore, .tau.=a/V.sub.a. In the case of a flat arrangement of the arc, its length may exceed the value of a several times at most. For example, 1.sub.a =.pi.a if the arc is a semicircular one. With 1.sub.a .tau.U/E, a relationship between the switching time .tau. and the turned-off voltage U in the case of electromagnetic switchgear is given by .tau.=U/kEV.sub.a where k=1.sub.a /a is the proportionality coefficient to relate the arc length and the path covered by the arc column, this coefficient being dependent upon the arc shape. In the case of a freely moving arc, the intensity E is dependent upon the arc velocity and current and is usually equal to 10 to 100 V/cm. With U=10 kV at V.sub.a .apprxeq.50 m/s, E.apprxeq.30 V/cm and k=.pi., the switching time .tau. becomes equal to 0.02 s, which gives the overall size of the arc extinction apparatus equal to 2a.apprxeq.2 m.
To reduce the overall size of the arc extinction apparatus and provide for better operating conditions and good arrangement of its components, electromagnetic switchgear is usually provided with an arc chamber which is a slit-shaped structure formed from plates made of electrically insulating material. The arc chamber operates to form the quenching arc and to determine the direction of its movement, provides for an increase the intensity of the electric field in the arc column by compressing and cooling the latter, and makes it possible to utilize magnetic circuits that help enhance magnetic blast. When a labyrinth (zigzag-like) slit is used the quenching arc can be lengthened additionally. However, the length of the arc can be increased using a labyrinth arc chamber only several times since the force F responsible for the movement of the arc column is decreased in this case with the result that switchgear is given a lower response.
To interrupt currents that are smaller in comparison to the rated current magnitude and that therefore result in a decrease in the effectiveness of magnetic blast, electromagnetic circuit breakers are usually provided with a self-blast air system.
Labyrinth arc chambers made it possible to develop electromagnetic circuit breakers rated, for example, for 10 to 20 kV and having the overall size of 1 m approximately and the switching time not exceeding 0.06 s. However, electromagnetic circuit breakers cannot find use at the present for switching higher voltages since their dimensions would become too large in this case along with a decreased switching time which is an important parameter of a circuit breaker.
The range of working voltages handled by electromagnetic circuit breakers can be increased by virtue of a helix-shaped breaking arc, the helix having a small value of the pitch .lambda..
Known in the art is a circuit breaker utilizing a helix-shaped quenching arc (cf. J. Miyachi, H. Naganawa, Spiral Arc in SF.sub.6 Facilitating DC Interruption, III International Conference on Gas Discharges, London, 1974, p. 521). In this circuit breaker, a free straight arc is formed between electrodes closed by a wire after the latter is exploded electrically or after the parting of the electrodes in an axial direction. The arc surrounded by a magnetic field applied in a longitudinal direction relative to the electrodes axis takes the form of a helix that expands in a radial direction and the voltage across the electrodes tends to rise in this case. However, the helix features a nonregular form due to the presence of a large number of random distortions of very diversified shapes and dimensions. This results in a condition where certain portions or turns of the arc column are caused to converge and a breakdown therefore takes place with the result that a sudden decrease in the voltage across the electrodes occurs, while the portions of the arc column brought together are shunted and disintegrated. This phenomenon basically applies to small-scale distortions of the arc column which tend to develop at a greater rate.
This arc is therefore difficult to utilize; a ratio between the arc length and the electrode spacing amounts to 10 to 12 with the arc diameter of 4 to 6 cm and the helix pitch of 3 cm approximately.
There is an arc extinction apparatus for switchgear (cf. German Pat. No. 330,268, cl. 23c 35.sup.05, 1919), which apparatus comprises two electrodes adapted to produce a quenching arc in the the from of a helix that expands in a radial direction.
In the described apparatus, the electrodes are mounted on a cylinder member made of an electrically insulating material and are bent to take the form of a helix. To protect the electrode turns from breakdown, the cylinder member has a helix-shaped partition made of an electrically insulating material and having the value of the pitch of its helix equal to the electrode helix pitch.
The expansion of the arc in a radial direction is attained due to the force of electromagnetic interaction between the tangential component of the arc current and the longitudinally oriented magnetic field formed by an arc-quenching coil disposed within the cylinder member.
When the electrodes are caused to move in opposite directions the parting of the electrodes takes place and a quenching arc is struck. The electromagnetic interaction between the radial component of the arc current at the areas adjacent the electrodes and a longitudinally oriented magnetic field causes the "winding" of the arc on to the helix-shaped electrodes with the result that the arc assumes the form of a helix.
The quenching arc expands in a radial direction and moves away from the surface of the cylinder member at a location where the helix partition does not give influence on the from of the arc column and on the direction in which the latter is moved and does not resist the occurrence of a breakdown between the adjacent turns of the helix arc. As a result, the arc can be lengthened within specific limits only and the working voltage at which the switchgear operates reliably amounts to a value of several kV, the proper overall size of the apparatus not exceeding 1 m in this case.
The described apparatus is therefore disadvantageous in that the working voltage is low due to a small length of the helix-shaped arc so produced, which is equal to one or more turns. In addition, the electrodes must follow a helix path and must have a helix shape, which results in complex design features of the associated switchgear.